One form of submerged pump system includes a vertical turbine pump which in use is submerged in water or other liquid within a well or vessel for the purpose of pumping the liquid upwardly through a pipe to a discharge point. A typical pump of this kind includes, as its essential components, a vertical stack of impellers mounted on a motor-driven vertical shaft and shrouded within housings, the lowermost housing having an inlet open to the water when the pump is submerged. When the shaft rotates, the impellers rotate within the housings thereby lifting water from the inlet to each successive impeller. The combined action of the impellers creates sufficient force to pump water to the top of the well through a vertical pipe, commonly referred to as the suction pipe, the lower end of which is connected to the uppermost housing. Such pumps are capable of pumping water from wells in which the water level is many hundreds of feet below the top of the well. Typically the motor is mounted at the top of the well with its output shaft coaxial with the impeller shaft, and the water discharge opening at the top of the well faces laterally and is connected to a conduit which conveys the water to a water system.
A pumping system which utilizes a submerged pump such as a vertical turbine pump typically includes a check valve installed between the discharge opening of the suction pipe and a conduit which conveys the water away from the opening. The check valve opens under the force of the pumped water so as to pass the water into the conveying conduit and closes under the force of water tending to flow in the opposite direction.
When the pump is not operating, that is when the impellers are not rotating, the portion of the suction pipe between the water level in the well and the discharge opening is filled with air. When the pump starts, this air must be exhausted to atmosphere and not be pushed by the force of the pump out the discharge, through the check valve and into the downstream water system. If the air enters the water system, various problems may result, including water hammer and pressure surges which are capable of causing damage to the pump, the piping in the water system, water meters and other sensitive water control devices. Typically, upon pump start-up, the air in the suction pipe is discharged to atmosphere through a valve, commonly referred to as an air and vacuum valve or simply an air valve, located between the discharge opening and the check valve. Such air valves are usually float valves which comprise a housing in communication with the suction pipe, a vent communicating with atmosphere and a buoyant float in the housing, the float rising to close the vent when water enters the housing due to operation of the pump. When the pump stops, water drains from the valve housing, allowing the float to move downwardly and open the vent, so that ambient air can re-enter and flow into the upper end of the suction pipe. The re-entering air allows the water in the suction pipe to flow back through the impellers into the well until the water level in the suction pipe is the same as the water level in the well. Allowing the water to return to water level is a necessary feature in order to prevent the creation of negative pressure (vacuum) in the suction pipe, because the negative pressure puts a strain on the pump shaft seals and various couplings and expansion joints which may be present. Also, the reverse flow of water causes the pump impellers to rotate in the reverse direction; this back-spin is an inherent disadvantage, because should the pump be called to start again while in reverse rotation, the resulting stress on the pump shaft could cause the latter to break.
Another conventional feature which can be incorporated into the pumping system so far described is an air throttling valve which is fitted to the vent opening of the air valve. A throttling valve is simply a flow restriction device which restricts the flow rate of venting air and thereby creates a back pressure on the rising column of water during start-up of the pump. This back pressure reduces the rate at which the water rises in the suction pipe thereby reducing the shock effect of the water on the valve-closing mechanism of the air valve and on the valve-opening mechanism of the check valve. The air valve, if its structural and operating characteristics are closely matched to the pump capacity and other variables of the pumping system, will itself compensate for the shock effect of the rising water and in such a situation no throttling valve would be necessary. As a practical matter, it is often useful to incorporate a throttling valve and then manually adjust the restricting effect of the latter to produce the desired degree of throttling for the particular system.
Conventional throttling valves have, however, an inherent disadvantage in that they also restrict the flow of re-entry air when the pump stops. The result is that the column of water in the suction pipe drops rather slowly, thus increasing the time period during which the impellers are caused to rotate in the reverse direction and during which the shaft seals and other components are under vacuum stress. The potential hazard of shaft breakage if the pump receives a start signal during this time period, as discussed previously, is thus increased. The present invention provides an improved throttling valve which allows essentially unrestricted flow of re-entry air, thus overcoming these disadvantages of conventional throttling valves.